Wire bonding is used to connect electrical contact points of different devices, or different electrical contact points of the same device.
FIG. 1 is a side view of a conventional wire bonding tool and an electronic flame-off (EFO) device. The wire bonding tool includes a horn 10 and a capillary 12 attached to the horn, and also other components such as an ultrasonic transducer (not shown) for generating bonding energy. The EFO device, comprising an EFO torch 14, is positioned adjacent to the capillary 12 of the wire bonding tool. The capillary 12 is configured to travel between a first height adjacent to the EFO torch 14 for sparking, and a second height for bonding.
Bonding wire (not shown) is threaded through a centrally-located hole in the capillary 12 and extends from the narrow tip of the capillary 12. The bonding tool is arranged over an electronic device to be bonded. The electronic device typically comprises a die 16 mounted onto a leadframe 18, and wire connections are usually made between the die 16 and the leadframe 18. A window clamp 20 clamps and secures the electronic device to a worktable. The window clamp 20 has an opening to expose a bonding area on the electronic device at which wire bonding is to be conducted, for access by the capillary 12.
During operation, bonding wire hangs from the tip of the capillary 12. The end of the wire is prepared for bonding by locating the EFO torch 14 at a predetermined distance from the end of the wire and emitting an electrical arc between the end of the wire and the EFO torch 14. The arc forms a molten ball at the end of the bonding wire. By varying the intensity and the duration of the electrical arc, the size of the ball that is formed can be adjusted to specific dimensions. The molten ball is then attached to the bonding surface by the capillary 12 using force and power. Thereafter, the wire is extended to another bonding surface to form a second bond.
When bonding wires made of reactive materials such as copper or aluminum are used for wire bonding, there is a tendency for oxidation of the molten ball to occur when the melted material reacts with oxygen in the atmosphere. Oxidation of the molten ball degrades the quality of the subsequent ball bond that is formed. Therefore, it is necessary to provide a shielding gas comprised of a relatively inert gas such as nitrogen or argon gas to cover and protect the wire during ball formation.
A simple way of supplying such shielding gas would be to position one or more gas nozzle tubes adjacent to the capillary 12 to envelope the end of the bonding wire during molten ball formation. The problem is that, if done in the open, this is not very effective and the results are inconsistent because much of the gas is able to immediately escape into the atmosphere.
An apparatus for containing shielding gas when supplying the gas to bonding wire is disclosed in U.S. Pat. No. 6,234,376 entitled “Supplying a Cover Gas for Wire Ball Bonding”. The apparatus includes a gas-containment tube to direct shielding gas to the bonding wire. Transverse in-line orifices are made in the tube to allow the capillary of the bonding tool to enter the gas-containing tube for ball formation, and then to pass through the tube to bond the ball onto a bonding surface.
Although the apparatus is effective for supplying a shielding gas to the bonding wire during ball bonding to protect the molten ball from oxygen in the atmosphere, the containment tube itself is an impediment to viewing the wire bonding process because the containment tube fully encloses the tip of the capillary and hides it from view. Thus, the disclosure teaches that the containment tube should be made of clear quartz glass, or less preferably heat resistant plastics, to allow observance of the ball forming operation. The set-up of the EFO torch relative to the capillary and the containment tube is also difficult because the tip of the EFO torch is located within the containment tube. Thus, the alignment between the EFO torch, capillary and containment tube can only be done with some difficulty by looking through the transparent tube, which also tends to become more opaque over time. Inaccurate alignment among the items can create reliability issues and an erroneous set-up may cause ineffectiveness of the apparatus.
It would therefore be desirable to develop an apparatus for supplying shielding gas to a bonding wire that does not fully enclose the tip of the capillary in order to make it easier to align the various components of the wire bonding tool and also to make it possible to directly view the ball forming operation.